In recent years, as more exceptional wells, extra-deep wells, and complex wells are drilled, the drilling of deep wells and extra-deep wells has been regarded as an important aspect in the future development of the drilling industry. Higher requirements have been put forward for the drilling fluid technology, and the existing drilling fluid additives can't fully meet the demand of the development of the drilling fluid technology anymore. Since there are more technical difficulties in drilling of deeper wells, in all countries around the world, the drilling depth and speed are deemed as important indicators of the well drilling technology. It is proven in practice that the quality of extra-deep well drilling fluids is of vital importance for the success of drilling of deep wells and extra-deep wells, drilling speed, exploration of deep oil and gas resources, and drilling cost. Viewed from the present situation of the research on deep well and extra-deep well drilling fluids, common deep well and extra-deep well drilling fluids are categorized into two broad categories: water-based drilling fluids and oil-based drilling fluids. Though oil-based drilling fluids have advantages such as resistant to high temperature, resistant to salinity and calcium contamination, and beneficial for well wall stability, etc. over water-based drilling fluids, they involve much higher preparation costs, may bring severe impacts on the ecological environment in the vicinity of the well site, and usually attain lower penetration rates than water-based drilling fluids. Water-based drilling fluids have advantages such as low cost, simple preparation, disposal and maintenance, wide availability and varieties of additives, and easy control of properties, etc.; therefore, water-based drilling fluids are dominant in China. In view of the shortcomings of water-based drilling fluids, it is a technical key to develop a water-based drilling fluid that has high performance and is stable for drilling of deep wells and extra-deep wells, as well as a technical challenge in drilling of deep wells and extra-deep wells in China and foreign countries.
The development from the tri-sulfonate drilling fluid developed in the early 1970s to the present polysulfonate drilling fluid systems is an inexorable development trend of deep-well drilling fluids. The majority or even the vast majority of deep-well drilling fluid systems can't function without polysulfonates. The action principle of polysulfonates is mainly resistance to damages of high temperature to the additives, and relevant additives are based on a prerequisite of improving the temperature-resistance capability and salinity-resistance capability. The temperature-resistance capability mainly lies in resistance to high temperature degradation.
However, existing deep-well high-temperature drilling fluids have the following major problems:    (1) The filtrate loss and wall building property of the high-temperature drilling fluid system is difficult to control: Since additives that contain chromium element were forbidden to use, sharply aggravated HTHP filtrate loss and aging of deep-well high-temperature drilling fluid systems (especially brine drilling fluids) has become a universal phenomenon, and that phenomenon is more prominent as the temperature and salinity increase. Therefore, developing effective temperature-resistant and salinity-resistant filtrate reducers (for reducing HTHP filtrate loss) is a key task in the drilling fluid field as well as a core problem for establishing high-temperature brine drilling fluid systems. In addition, after the HTHP filtrate loss and thermostability problems of low-density brine drilling fluids are solved, when such low-density brine drilling fluids are weighted to 1.50 g/cm3 or higher density, the HTHP filtrate loss will be increased severely; when the density is increased to be higher than 2.0 g/cm3, the HTHP filtrate loss will be multiplied. Consequently, more kinds of filtrate reducers have to be used, and the doses of the filtrate reducers have to be increased further. As a result, the kinds and doses of high-temperature filtrate reducers for drilling fluid are increased greatly, and the approaches for development of temperature-resistant and salinity-resistant filtrate reducers are limited.    (2) The rheological property of high-density drilling fluid systems are difficult to control: To ensure high-temperature drilling fluids have low HTHP filtrate loss, the kinds and doses of high-temperature filtrate reducers for drilling fluid have to be increased greatly in the systems. Consequently, the viscosity of the base mud of drilling fluid is increased severely; in addition, owing to the existence of a large quantity of barite granules after the base mud is weighted, the viscosity is further increased severely. If the quantity of soil in the drilling fluid is controlled improperly, the viscosity and shearing strength of the drilling fluid will be increased after the drilling fluid is aged at a high temperature, resulting in loss of fluidity (gelatinization and setting at high temperature). In addition, the temperature-resistant and salinity-resistant viscosity reducers being developed continuously mainly act on clay, but the content of clay in high-density drilling fluid systems is very limited. Consequently, it is difficult to attain a good viscosity breaking effect.
Though extensive researches on temperature-resistant and salinity-resistant drilling fluids have been made in China presently, but existing high temperature-resistant high-density brine drilling fluid systems still can't fully meet the performance expectations. Therefore, the “Research on High-Density Drilling Fluid Systems (≤2.30 g/cm3) Resistant to 200° C. or Higher Temperature” has been listed as a key special research task and subject in “the Thirteenth Five Year Plan” of the state for development of the petroleum and gas industry.
Deep-well high-density water-based drilling fluid systems become more complex as the temperature is increased, the kinds of additives are increased, and the doses of the additives are increased; especially, it has become a consensus in the mud industry in China and foreign countries that it is very difficult to control the HTHP filtrate loss of a drilling fluid to be lower than 15 ml after the drilling fluid is aged at a high temperature above 180° C. and a variety of filtrate reducers have to be used and the doses of the filtrate reducers have to be increased greatly to attain that goal.
Owing to the fact that there are many kinds of additives for high-temperature brine drilling fluids that work at 180° C. or higher temperatures, the doses of the additives are very high and the systems are very complex, it is difficult handle the additives once their performance become instable during use. The results of statistics in foreign countries have demonstrate that the service temperature of a drilling fluid is related with the consumption of additives (organic); specifically, when the temperature is increased from 93° C. to 121° C., the consumption of the additives will be increased by 50%; when the temperature is increased from 121° C. to 148.9° C., the consumption of the additives will be increased by 100%; when the temperature is increased from 148.9° C. to 176.7° C., the consumption of the additives will be increased by 100%.
Likewise, the high-temperature high-density brine drilling fluids produced domestically in China also have the characteristics described above. The results of statistics have indicated that usually 10 kinds of organic additives exist in a high-density (2.00 g/cm3) under-saturated brine drilling fluid system that is resistant to 200° C. high temperature, the total dose of filtrate reducers is higher than 25%, and the total dose of organic additives in the system is even much higher than 30%.